Sintering furnace, method of manufacturing sintered objects, and sintered objects

ABSTRACT

A sintering furnace for sintering an object to be sintered formed of ceramics, fine ceramic materials, etc. to produce a sintered object and a method therefor. An insulating wall ( 28 ) and an inner shell ( 25 ) define a sintering chamber ( 16 ) for sintering an object to be sintered ( 10 ). Thermal equilibrium is maintained between the inner shell ( 25 ) and the object to be sintered ( 10 ), and the object to be sintered ( 10 ) is completely pseudo-adiabatically isolated to achieve more uniform and small energy consuming sintering. The thickness of the insulating wall ( 28 ) increases gradually from an inlet ( 20 ) toward an outlet ( 21 ). The object to be sintered ( 10 ) is fed by a carriage provided in a sintering furnace from the inlet ( 20 ) to the outlet ( 21 ) in the sintering chamber ( 16 ). Thereby, a temperature distribution corresponding to a plurality of processes can be easily formed in one furnace to sinter continuously the object to be sintered ( 10 ) in the furnace.

BACKGROUND OF THE INVENTION

The present invention relates to a sintering furnace for sintering anobject to be sintered formed of ceramics, fine ceramic materials, etc.to produce a sintered object, a method of manufacturing a sinteredobject, and a sintered object.

Conventionally, an electric furnace or a gas furnace has been used as asintering furnace for sintering an object to be sintered. However, sincethe temperature in a furnace must be raised gradually in order not togenerate any temperature differences between the surfaces and theinteriors of the object to be sintered in the case of such furnaceswhere the object to be sintered is heated from outside, there has been aproblem that sintering time is liable to be longer.

Accordingly, in order to solve such a problem, Japanese Examined PatentPublication No. Sho 58-23349, Japanese Laid-Open Patent Publication No.Hei 3-257072 and Japanese Laid-Open Patent Publication No. Hei 6-87663propose various sintering furnaces using microwaves. Microwaves areabsorbed uniformly both onto the surfaces and into the interiors of theobjects to be sintered. Therefore, there is only a little fear that anytemperature differences occur during heating between the surfaces andinteriors of the object to be sintered. Consequently, the rate oftemperature rise can be increased to shorten the time necessary for thesintering to a large extent as well as uniform sintering can beaccomplished. Sintering the objects to be sintered with microwaves isexpected as such a technology that decreases energy necessary for thesintering as well as increases productivity of producing sinteredobjects, especially ceramics for industrial use.

The inventors of the present invention found that when sintering isconducted using microwaves, a pseudo-adiabatic space completelyinsulating an object to be sintered is created by surrounding the objectto be sintered with a thermal insulating material which has anequivalent microwave absorption property to that of the object to besintered. In this case, occurrence of thermal gradient in the object tobe sintered due to radiation cooling can be restrained and more uniformsintering can be accomplished.

However, since energy of the microwaves is absorbed and consumed notonly in the object to be sintered but also in the insulating material inthe case of sintering the object to be sintered surrounded by the aboveinsulating material, the amount of energy necessary for the sinteringincreases significantly.

In order to restrain the amount of energy consumed in the insulatingmaterial, it is necessary to make the insulating material thinner todecrease its weight and heat capacity. However, if the insulatingmaterial is made thinner, the amount of thermal energy lost by heatconduction out of the insulating material becomes larger to such adegree that cannot be neglected, compared to the amount of thermalenergy given by the microwaves to the insulating material. Accordingly,there occurs a large temperature difference between the inside surfaceof the insulating material and the object to be sintered. Consequently,the above assumption of pseudo-adiabatic space will not be establishedresulting in occurrence of thermal gradient in the object to be sintereddue to radiation cooling.

Therefore, a first object of the present invention is to provide asintering furnace where occurrence of thermal gradient in an object tobe sintered due to radiation cooling can be restrained while attemptingto reduce energy necessary for sintering the object to be sintered, anda method of manufacturing an object to be sintered, and a sinteredobject.

For intending mass production of sintered objects, a tunnel typecontinuous sintering furnace is preferable where a plurality ofprocesses can be carried out continuously. In the continuous sinteringfurnace, it is necessary to form an appropriate temperature distributionin the furnace by changing the temperature in the furnace in thedirection of carrying the objects to be sintered. The reason is becauseeach process (for example, drying, preliminary sintering, mainsintering) of the sintering processes must be done in the particulartemperature region within the furnace corresponding thereto.

However, in the case of the sintering with microwaves, formation of aproper temperature distribution in the furnace, which is a continuouscavity, is difficult, because electric power density of the microwavesis dispersed and uniformized through repetition of multiple reflectionof microwaves within the furnace.

Therefore, a second object of the present invention is to provide acontinuous sintering furnace where a temperature distributioncorresponding to a plurality of processes can be easily formed in onefurnace and an object to be sintered can be continuously sintered withmicrowaves in the furnace, and a method of manufacturing an object to besintered, and a sintered object.

DISCLOSURE OF THE INVENTION

According to an embodiment of the present invention, a sintering furnacefor sintering objects to be sintered with microwaves is provided. Thesintering furnace comprises an inner shell, which can heat itself withmicrowaves, and a microwave generator. The inner shell defines asintering chamber and the objects to be sintered are disposed in thesintering chamber. The microwave generator radiates microwaves to theobjects to be sintered via the inner shell. The amount of heat generatedwith the microwaves per unit volume of the inner shell is larger thanthe amount of generated heat per unit volume of the objects to besintered. The temperature of the inside surface of the inner shell issubstantially the same as the surface temperature of the objects to besintered.

According to another embodiment of the present invention, provided is amethod of manufacturing sintered objects where the sintered objects areformed by radiating microwaves to objects to be sintered. The methodcomprises the steps of: i) providing an inner shell which can heatitself with microwaves and which defines a sintering chamber; ii)disposing objects to be sintered within the sintering chamber, theamount of heat generated with the microwaves per unit volume of theinner shell being larger than the amount of generated heat per unitvolume of the objects to be sintered; iii) forming sintered objects byradiating microwaves with a microwave generator to the objects to besintered via the inner shell in order to make the temperature of theinside surface of the inner shell substantially the same as the surfacetemperature of the objects to be sintered.

According to still another embodiment of the present invention, asintered object obtained through the above method is provided.

According to another embodiment of the present invention, provided is acontinuous sintering furnace for sintering objects to be sintered withmicrowaves. The continuous sintering furnace comprises an insulatingwall permeable to microwaves, a microwave generator and a feedingsystem. The insulating wall defines a sintering chamber and the objectsto be sintered are disposed in the sintering chamber. The microwavegenerator radiates microwaves to the objects to be sintered via theinsulating wall. The feeding system feeds the objects to be sinteredinto the sintering chamber. The temperature within the sintering chamberis changed in order to correspond to the sintering process of theobjects to be sintered in the direction of feeding the objects to besintered.

According to yet another embodiment of the present invention, providedis a method of manufacturing sintered objects where the sintered objectsare formed by radiating microwaves to objects to be sintered. The methodcomprises the steps of: i) providing an insulating wall permeable tomicrowaves, the insulating wall defining a sintering chamber; ii)disposing the objects to be sintered within the sintering chamber by afeeding system; iii) forming the sintered objects by radiatingmicrowaves with a microwave generator via the insulating wall to theobjects to be sintered carried into the sintering chamber so that thetemperature within the sintering chamber is changed in order tocorrespond to the sintering process of the objects to be sintered in thefeeding direction of the objects to be sintered.

According to still another embodiment of the present invention, sinteredobjects obtained through the above method are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional plan view showing an embodiment of thesintering furnace of the present invention.

FIG. 2 is an enlarged schematic sectional plan view showing thesintering chamber.

FIG. 3 is a schematic sectional side view showing a first embodiment ofthe continuous sintering furnace.

FIG. 4 is an enlarged schematic sectional plan view showing thesintering furnace in FIG. 3.

FIG. 5 is a graph showing the temperature dependence of complexdielectric loss of the insulating wall.

FIG. 6 is a schematic plan view showing a second embodiment of thecontinuous sintering furnace.

DETAILED DESCRIPTON OF THE PREFERRED EMBODIMENTS

In the following, the embodiments of the present invention are describedon the basis of the drawings in order to describe the present inventionin more detail. Unless otherwise mentioned, like reference numbers referto like members throughout the drawings.

(Sintering Furnace)

FIG. 1 is a schematic sectional view showing the sintering furnace of afirst embodiment. The sintering furnace is used to manufacture sinteredobjects by sintering an object to be sintered 10. The object to besintered 10 is composed of an object molded into a predetermined shapefrom a ceramic material or a fine ceramic material. The object to besintered 10 can be any one selected from a molded object, an unglazedmolded object, a glazed molded object, and an article obtained byglazing an unglazed molded object.

The sintering furnace comprises a chamber 11 composed of a closedcontainer. The chamber 11 is able to reflect microwaves at least on theinside surface thereof. In the present embodiment, the chamber 11 isformed in the shape of a square box made of stainless steel with 2 m inlength, 1.1 m in width, and 1.1 m in height.

Microwave oscillators 12 as the microwave generator are connected to thechamber 11 via waveguides 13. Microwaves are radiated into the chamber11 via the waveguides 13. The frequency of the microwaves is preferably0.9 to 100 GHz, more preferably 0.9 to 10 GHz, most preferably 2.45 GHz.The frequency lower than 0.9 GHz is not preferable because wavelengthbecomes very long and brings about reduction in absorptance ofmicrowave. On the contrary, the frequency higher than 100 GHz is notpreferable because an expensive microwave oscillator 12 is necessary.When the frequency of the microwaves is 2.45 GHz, the microwaveoscillator 12 can be rather small and low in price. In the presentembodiment, used are six microwave oscillators 12 (only four of them areshown in FIG. 1) which output 2.45 GHz microwave (output 1.5 kW).

An auxiliary insulating wall 27 defines space within the chamber 11. Theauxiliary insulating wall 27 is adiabatic as well as permeable tomicrowaves. Both a main insulating wall 26 and an inner shell 25disposed within the main insulating wall 26 define a sintering chamber16 in the space, which is defined by the auxiliary insulating wall 27.The inner shell 25, the main insulating wall 26 and the auxiliaryinsulating wall 27 compose a bracket surrounding the object to besintered 10. The volume of the sintering chamber 16 is preferably 0.3 to0.6 m³.

The main insulating wall 26 is adiabatic as well as permeable tomicrowaves. A material having adiabatic performance such as aluminafiber, alumina foam is given as a material for forming the maininsulating wall 26.

On the other hand, the inner shell 25 heats itself with microwaves. Theamount of heat generated with the microwaves per unit volume of theinner shell 25 must be essentially larger than the amount of generatedheat per unit volume of the object to be sintered 10 and may preferablybe equal to or smaller than 2 times thereof. As a material for formingthe inner shell 25 given are mullite based materials, silicon nitridebased materials and alumina, and they are provided for selectiondepending on the object to be sintered 10. Further, a metal oxide suchas magnesia, zirconia or iron oxide, or an inorganic material such assilicon carbide all of which have large microwave absorptance can beadded in a small amount to the material for forming the inner shell 25.The thickness of the inner shell 25 is preferably in a range of 1 to 2mm.

Further, the sintering furnace comprises a microwave stirring system forstirring microwaves irradiated into the chamber 11. The microwavestirring system includes a rotary shaft 17 extending inward from theinside surface of the chamber 11, a plurality of stirring vanes 18supported by the rotary shaft 17 and a driving motor 19 for rotating thestirring vanes 18 around the rotary shaft 17 on its axis.

Next, the manufacturing method of sintered objects using the abovesintering furnace will be described.

When the sintered object is manufactured, an object to be sintered 10 isfirst made through molding ceramic material or fine ceramic materialinto the predetermined shape. The object to be sintered 10 is placed inthe sintering chamber 16. Subsequently, the microwave oscillator 12 isactuated to radiate microwaves into the chamber 11. The microwavesradiated into the chamber 11 are transmitted through the main insulatingwall 26 and the auxiliary insulating wall 27 and absorbed into the innershell 25 and the object to be sintered 10 to be converted into thermalenergy. This increases the temperature of the inner shell 25 and theobject to be sintered 10.

Since the thickness of the inner shell 25 of the present embodiment isthinner than that of conventional one, there is a possibility that theamount of thermal energy lost due to heat conductivity from the innershell 25 to the outside becomes larger to such a degree that cannot beneglected compared with the amount of thermal energy gained by the innershell 25 with the microwaves. However, the amount of heat generated perunit volume of the inner shell 25 is larger than the amount of heatgenerated per unit volume of the object to be sintered 10. Therefore, bythis difference between the amount of heat generated in the inner shell25 and that in the object to be sintered 10, compensated is thedifference between the inside surface temperature of the inner shell 25and the surface temperature of the object to be sintered 10.Consequently, the thermal equilibrium between the inner shell 25 and theobject to be sintered can be maintained to make the inside surfacetemperature of the inner shell 25 substantially equal to the surfacetemperature of the object to be sintered 10. This shows that the objectto be sintered 10 is completely pseudo-adiabatically insulated. The factthat the inside surface temperature of the inner shell 25 issubstantially the same as the surface temperature of the object to besintered 10 refers to that the temperature difference between the two issuch a difference as not to create any harmful thermal strain. Thetemperature difference is preferably equal to or less than 20° C.

Further, since the inner shell 25 is heated while maintaining thethermal equilibrium with the object to be sintered 10, the thermalenergy lost through radiation from the object to be sintered 10 iscanceled by the thermal energy radiated from the inside surface of theinner shell 25 to the object to be sintered 10. At this time, theradiation loss of the object to be sintered 10 becomes zero inprinciple. That is, the sintering chamber 16 acts as a closed space forthe object to be sintered 10, which is pseudo-adiabatically completelyinsulated. Thus, the occurrence of thermal gradient due to radiationcooling in the object to be sintered 10 is restrained.

According to a theoretical analysis, as shown in FIG. 2, when it isassumed that the object to be sintered 10 of one dielectric issurrounded at an appropriate distance apart by the inner shell 25 of theother dielectric, thermal conduction equations can be written as thefollowing equations (1) and (2).∂θ₁ /θt=κ₁(∂²θ₁ /∂x ²+∂²θ₁ /∂y ²+∂²θ₁ /∂z ²)+σ(θ₂ ⁴−θ₁ ⁴)+2πf/(c ₁ρ₁)E²∈₀∈_(r1) tan δ₁  (1)∂θ₂ /θt=κ₁(∂²θ₂ /∂x ²+∂²θ₂ /∂y ²+∂²θ₂ /∂z ²)+σ(θ₁ ⁴−θ₂ ⁴)+2 90 f/(c₂ρ₂)E ²∈₀∈_(r2) tan δ₂  (2)

Where θ is the temperature; κ is the heat conductivity; c is thespecific heat; ρ is the density; t is the time; x, y, z are thepositions; σ is the Stefan-Boltzmann constant; f is the frequency; E isthe field intensity; ∈_(r) is the relative permittivity of a material;∈₀ is the permittivity of vacuum; and δ is the loss angle. The suffix“1” refers to the object to be sintered 10, and the suffix “2” refers tothe inner shell 25.

In an ideally adiabatic state, the difference between thermal incomingsand outgoings through radiation, heat conduction and heat transfer isset at zero on the surface of the object to be sintered 10. Such a stateis realized when the temperature of the surface of the object to besintered 10 is equal to that of the inside surface of the inner shell25, namely at thermal equilibrium. In other words, since there is noenergy loss from the surface of the object to be sintered 10, that is,no heat flux from the interior of the object to be sintered 10 to thesurface, the temperature gradient ∂θ₁/∂x, ∂θ₁/∂y, ∂θ₁/∂z becomes zero.

Consequently, the above equation (1) is expressed as follows:∂θ₁ /∂t=2πf/(c ₁ρ₁)E ²∈₀∈_(r1) tan δ₁   (1′)The above equation (2) is also expressed as follows:∂θ₂ /∂t=P _(rf)/(c ₂ρ₂)∈_(r2) tan δ₂ −P _(loss)/(c ₂ρ₂)  (2′)

Where X=0, ∂θ₁/∂t=∂θ₂/∂t, θ₁=θ₂. Further, the inner shell 25 is a closedspace or equivalent to a closed space. In order that the thermalequilibrium may be established at the inside surface of the inner shell25, the incomings and outgoings of energy at the inside surface of theinner shell 25 must be zero, namely ∂θ/∂x=0 must be held true at theinside surface of the inner shell 25. Therefore, the condition, whichsatisfies the equation (2′), is expressed in the following equation (3):∈_(r1) tan δ₁ /c ₁ρ₁=∈_(r2) tan δ₂ /c ₂ρ₂ −P _(loss) /P _(rf)  (3)and, wherein P_(rf) (=2πf E² ∈₀) means electric power density ofmicrowave, and P_(loss) means energy lost outward from the inner shell25.

The present embodiment described in detail above has the followingeffects.

The thermal energy lost through radiation from the object to be sintered10 is canceled by the thermal energy radiated from the inside surface ofthe inner shell 25, and the radiation loss of the object to be sintered10 is zero in principle. Therefore, the occurrence of thermal gradientdue to radiation cooling in the object to be sintered 10 is restrained,so that more uniform sintering can be carried out in comparison with theconventional sintering furnaces with microwave. Thereby, the occurrenceof strains and cracks in the object to be sintered 10 can be restrained.

The amount of heat generated with microwaves per unit volume of theinner shell 25 is larger than the amount of heat generated with thoseper unit volume of the object to be sintered 10. Therefore, the weightand the heat capacity of the inner shell 25 can be turned small bymaking the thickness thereof thinner while maintaining the thermalequilibrium between the object to be sintered 10 and the inner shell 25.Thus, the reduction of energy amount necessary for sintering the objectto be sintered 10 can be realized by restraining the amount of energyconsumed within the inner shell 25.

The heat loss of the inner shell 25 can be effectively suppressed by thefact that the main insulating wall 26 having the adiabatic performanceand the permeability to microwave is provided on the outer surface ofthe inner shell 25.

The sintering furnace includes a plurality of microwave oscillators 12and also a plurality of incident holes through which the microwavesoutput from the microwave oscillator 12 are transmitted into the chamber11. Therefore, it can be restrained that the spotted uneven sinteringcomes out due to concentration of electric field on only a part of theobject to be sintered 10.

(Continuous Sintering Furnace)

Continuous Sintering Furnace of the First Embodiment

FIG. 3 is a schematic sectional side view showing a first embodiment ofthe continuous sintering furnace. FIG. 4 is an enlarged schematicsectional plan view showing the sintering furnace in FIG. 3. Thesintering furnace shown in these figures is used to manufacture asintered object by continuously sintering an object to be sintered.

The continuous sintering furnace comprises a chamber 11 in the shape ofa tunnel extending linearly. The chamber 11 is able to reflectmicrowaves at least on the inside surface thereof. The chamber 11 ismade of stainless steel. As shown in FIG. 4, openings are disposed atboth end portions of the chamber 11, and one is an inlet 8 (the openingon the left-hand side in FIG. 4) and the other is an outlet 9 (theopening on the right-hand side in FIG. 4).

As shown in FIG. 3, like the furnace in FIG. 1, microwave oscillators 12as the microwave generator are connected via waveguides 13 to thechamber 11. The microwaves output from the microwave oscillator 12 areradiated into the chamber 11 via the waveguide 13. The frequency of themicrowaves is preferably 0.9 to 100 GHz, more preferably 0.9 to 10 GHz,most preferably 2.45 GHz, as in the case of the sintering furnace inFIG. 1.

In FIG. 4, an insulating wall 28 defines a sintering chamber 16extending linearly in the longitudinal direction of the chamber 11.Openings are disposed at both end portions of the insulating wall 28,and one is an inlet 20 (the opening on the left-hand side in FIG. 4) andthe other is an outlet 21 (the opening on the right-hand side in FIG.4).

The insulating wall 28 is adiabatic as well as permeable to microwaves.The insulating wall 28 is configured in order for the thickness thereofto increase gradually from the inlet 20 to the outlet 21. A materialhaving adiabatic performance such as alumina fiber, alumina foam isgiven as a material for forming this insulating wall 28. In the presentembodiment, the insulating wall 28 includes a first insulating wall 26and a second insulating wall 27.

Preferably, within the insulating wall 28 provided is an inner shell 25,which heats itself with microwaves. The amount of heat generated withthe microwaves per unit volume of the inner shell 25 is preferablylarger than the amount of heat generated per unit volume of the objectto be sintered 10 and is equal to or smaller than 2 times thereof. As amaterial for forming the inner shell 25 given are mullite basedmaterials, silicon nitride based materials and alumina, and they areprovided for selection depending on the object to be sintered 10.Further, a metal oxide such as magnesia, zirconia or iron oxide, or aninorganic material such as silicon carbide all of which have largemicrowave absorptance can be added in a small amount to the material forforming the inner shell 25. The thickness of the inner shell 25 ispreferably 1 to 2 mm.

Further, a feeding system is provided in the continuous sinteringfurnace for feeding the object to be sintered 10 from the inlet 20 tothe outlet 21 in the sintering chamber 16. In the present embodiment,the feeding system includes a carriage 22. As shown in FIG. 3, thecarriage 22 has a mount portion 22 a for mounting the object to besintered 10 thereon and rollers 22 b coupled with the mount portion 22 afor moving the carriage. The carriage 22 not only transfers the objectto be sintered 10 within the sintering chamber 16 but also feeds themfrom the inlet 8 of the chamber 11 to the inlet 20 of the sinteringchamber 16 and feeds them from the outlet 21 of the sintering chamber 16to the outlet 9 of the chamber 11. The feeding of the object to besintered 10 by the carriage 22 is preferably performed at a constantfeed speed.

The manufacturing method of a sintered object using the above continuoussintering furnace will be described below.

When a sintered object is manufactured, the object to be sintered 10 isfirst made through molding ceramic material or fine ceramic materialinto the predetermined shape. The object to be sintered 10 is disposedon the mount portion 22 a of the carriage 22, and is fed into thesintering chamber 16 through the inlet 20 with the carriage 22. Next,the microwave oscillator 12 is actuated to radiate microwaves into thechamber 11. The incident microwaves are transmitted through theinsulating wall 28 and absorbed into the inner shell 25 and the object10 to be converted into thermal energy resulting in the temperature riseof both the inner shell 25 and the object to be sintered 10.

In the present embodiment, the thickness of the insulating wall 28increases gradually from the inlet 20 toward the outlet 21 and theadiabatic effect of the insulating wall 28 also increases from the inlet20 toward the outlet 21. Therefore, the temperature within the sinteringchamber 16 increases from the inlet 20 toward the outlet 21.Consequently, feeding of the object to be sintered 10 from the inlet 20to the outlet 21 means gradual feeding of the object to be sintered 10from the low temperature region to the high temperature region.

FIG. 5 is a graph showing the temperature dependence of complexdielectric loss of the insulating wall 28. As shown in the figure, thecomplex dielectric loss of the insulating wall 28 is almost proportionalto a temperature up to several hundred degrees Celsius and increasesexponentially at the temperature region higher than that.

The present embodiment described in detail above has the followingeffects.

The temperature within the sintering chamber 16 is set to increase fromthe inlet 20 toward the outlet 21 by making the thickness of theinsulating wall 28 increase gradually from the inlet 20 to the outlet21. Therefore, each process step of the object to be sintered 10 such asdrying, preliminary sintering, main sintering, etc. in the sinteringprocess of the object to be sintered 10 can be executed sequentially ata proper temperature. Thus, the sintered object can be manufactured bycontinuously sintering the object to be sintered 10 in a singlesintering furnace.

The thickness of the insulating wall 28 varies in the feeding directionof the object to be sintered 10. This makes it easy to form a particulartemperature distribution in the sintering chamber 16.

The object to be sintered 10 is surrounded in the sintering chamber 16by the inner shell 25 which can heat itself with microwaves. Since theamount of thermal energy gained in the inner shell 25 by microwaves issufficiently larger than the amount of thermal energy lost by conductionfrom the inner shell 25, the heat equilibrium can be maintained betweenthe inside surface of the inner shell 25 and the object to be sintered10. Consequently, the object to be sintered are pseudo-adiabaticallycompletely isolated. Thereby, occurrence of thermal gradient in theobject to be sintered 10 due to radiation cooling can be restrained andmore uniform sintering can be accomplished.

Continuous Sintering Furnace of the Second Embodiment

A continuous sintering furnace of the second embodiment of the presentinvention will be described in detail on the basis of the drawingsfocusing on the differences from the first embodiment.

FIG. 6 is a schematic plan view showing the second embodiment of thecontinuous sintering furnace. A chamber 11 is formed in a circular arcshape or a C-shape, and a sintering chamber 16 is also formed in acircular arc shape or a C-shape corresponding to the former.

Further, this continuous sintering furnace includes a furnace bed 23 inthe shape of a disk. The furnace bed 23 can rotate about the centerpoint C. An object to be sintered 10 is disposed on the furnace bed 23.The object to be sintered 10 is fed from an inlet 20 to an outlet 21 inthe sintering chamber 16 by the rotation of the furnace bed 23. Afeeding system in the second embodiment includes the furnace bed 23 anda driving system (not shown) such as a motor for driving the furnace bed23.

The present embodiment has the following effects.

The linear sintering chamber 16 having the same lengths is affected bythe portions with different temperatures of an insulating wall 18.However, the sintering chamber 16 in the circular arc shape or a C-shapeis affected by the smaller region of the insulating wall 18 than that ofthe linear sintering chamber 16. The inner area of the insulating wall28, which can be seen in the feeding direction from the object to besintered 10 in the same region of the sintering chamber 16, is small.Therefore, occurrence of spotted uneven sintering on the object to besintered, due to the heat transferred from the portions with differenttemperatures of the insulating wall 28, can be restrained.

Since the feeding system includes the furnace bed 23 and a drivingsystem for driving the furnace bed 23 to rotate, manufacturing issimple.

It is apparent to those skilled in the art that the present inventioncan be embodied in various different specific modes without departingfrom the spirit or the scope of the present invention. Particularly, itis to be understood that the present invention can be embodied in thefollowing modes.

The sintering furnace may further comprise a pre-treatment chamber fordrying or subjecting the object to be sintered 10 to unglazed baking inadvance. In this case, the pre-treatment chamber is disposed to be inparallel with the sintering chamber 16. The object to be sintered 10disposed in the pre-treatment chamber is dried or subjected to unglazedbaking with outward radiation heat or transmitting microwaves originatedfrom microwaves irradiated into the sintering chamber by the microwavegenerator 12. By this procedure, thermal efficiency in drying or bakingthe object to be sintered 10 with no glaze can be enhanced.

The adiabatic performance or microwave absorptance of the insulatingwall 28 can be changed in the feeding direction of the object to besintered 10 by forming a portion of the insulating wall 28 with amaterial different from the other portion. Alternatively, the adiabaticperformance or microwave absorptance of the insulating wall 28 can bechanged in the feeding direction of the object to be sintered 10 byforming a portion of the insulating wall 28 in a density different fromthe other portion. In either case, the temperature in the sinteringchamber 16 can be changed in the feeding direction.

The insulating wall 28 may be 1-layered or more than 2-layered inaddition to 2 layers.

The feeding system can be changed to such a feeding system that includesa driving system having a belt conveyer and a motor and the like fordriving the belt conveyer. Further, in the continuous sintering furnaceof the second embodiment, the feeding system can be replaced with afeeding system having a carriage 22 as in the first embodiment.

In place of increasing the thickness of the insulating wall 28 graduallyfrom the inlet 20 to the outlet 21, the portion with a constantthickness or the portion with decreasing thickness may be partiallyprovided. Further, the variation in thickness may not only be continuousbut also stepwise.

In place of increasing the temperature in the sintering chamber 16gradually from the inlet 20 to the outlet 21, the portion with aconstant temperature or the portion with decreasing temperature may bepartially provided in the feeding direction. Further, the variation intemperature in the sintering chamber 16 may not necessarily becontinuous but stepwise.

EXAMPLES Example 1

A sintered object (ceramics) was obtained by sintering the object to besintered 10 (weight: 10 kg, average thickness: 5 mm) made of a ceramicmaterial using the sintering furnace of the embodiment shown in FIG. 1.

In this Example 1, the inner shell 25 was formed of mullite-basedporcelain and the main insulating wall (outer shell) 26 was formed of analumina-fiber board. Additionally, the inner shell 25 was 8 mm thick and45 kg in weight, and the main insulating wall 26 was 40 mm thick and 5kg in weight. The physical properties of the inner shell 25, the maininsulating wall 26 and the object to be sintered 10 are shown inTable 1. The penetration depth represents an entering depth at which theelectric power density of microwaves attenuates half of that in eachmaterial.

TABLE 1 Heat Dielectric Penetration conductivity Relative loss DensitySpecific heat depth Material (kW/m ° C.) permittivity (tan δ) (kg/m³)(kJ/kg ° C.) (m) Inner shell Mullite- 2.1 × 10⁻³ 6.5 1.5 × 10⁻³ 1.7 ×10³ 0.8 3.4 series porcelain Outer shell Alumina- 0.1 × 10⁻³ 9.5   3 ×10⁻⁵ 0.2 × 10³ 0.11 150 fiber (0.2/3.6 × 10⁻⁴⁾ board Object to beCeramics 1.18 to 1.59 × 10⁻³ 6 1.5 × 10⁻³ 2 to 3 × 10³ 0.88 3.5 sinteredmaterial

Example 2

A sintered object (ceramics) was obtained by sintering the object to besintered 10 (weight: 10 kg, average thickness: 5 mm) made of a ceramicmaterial using the sintering furnace of the embodiment shown in FIG. 1.

In this Example 2, the inner shell 25 was formed of mullite-based cementadded with 0.1% of iron oxide (FeO) and the main insulating wall (outershell) 26 was formed of an alumina-fiber board. Additionally, the innershell 25 was 2 mm thick and 5 kg in weight, and the main insulating wall26 was 40 mm thick and 5 kg in weight. The physical properties of theinner shell 25, the main insulating wall 26 and the object to besintered 10 are shown in Table 2.

TABLE 2 Heat Dielectric Penetration conductivity Relative loss DensitySpecific heat depth Material (kW/m ° C.) permittivity (tan δ) (kg/m³)(kJ/kg ° C.) (m) Inner shell Mullite- 2.1 × 10⁻³ 6.5 1.8 × 10⁻³ 1.7 ×10³ 0.8 3.0 series cement + FeO Outer shell Alumina- 0.1 × 10⁻³ 9.5   3× 10⁻⁵ 0.2 × 10³ 0.11 150 fiber (0.2/3.6 × 5 × 10⁻⁴) board Object to beCeramics 1.18 to 1.59 × 10⁻³ 6 1.5 × 10⁻³ 2 to 3 × 10³ 0.88 3.5 sinteredmaterial

Example 3

A sintered object was obtained by sintering the object to be sintered 10(weight: 10 kg, average thickness: 5 mm) made of high purity (99%)alumina with the sintering furnace of the embodiment shown in FIG. 1.

In this Example 3, the inner shell 25 was formed out of alumina addedwith 1 mol % of zirconia and the main insulating wall (outer shell) 26was formed of an alumina-fiber board. Additionally, the inner shell 25was 1 mm thick and 0.2 kg in weight, and the main insulating wall 26 was40 mm thick and 5 kg in weight. The physical properties of the innershell 25, the main insulating wall 26 and the object to be sintered 10are shown in Table 3.

TABLE 3 Heat Dielectric Penetration conductivity Relative loss DensitySpecific heat depth Material (kW/m ° C.) permittivity (tan δ) (kg/m³)(kJ/kg ° C.) (m) Inner shell Alumina + 2.4 × 10⁻³ 9.5 0.8 × 10⁻³   3 ×10³ 0.8 3.0 zirconia Outer shell Alumina- 0.1 × 10⁻³ 9.5 3 × 10⁻⁵ 0.2 ×10³ 0.11 150 fiber (0.2/3.6 × 5 × 10⁻⁴) board Object to be High 2.4 ×10⁻³ 6 0.5 × 10⁻³   3 × 10³ 0.8 3.5 sintered purity alumina

The examples and the embodiments of the present invention areillustrative and not intended to restrict the present invention. Thepresent invention should not be limited to the details described in thepresent specification and can be changed without departing from thescope of the appended claims and the equivalents thereof.

As described above, the sintering furnace and the manufacturing methodof sintered object according to the present invention are useful formanufacturing the sintered objects by sintering objects to be sinteredmade of ceramic materials or fine ceramic materials, and areparticularly suitable for executing not only a single step but also aplurality of steps of the manufacturing process of the objects to besintered.

1. A continuous sintering furnace for sintering an object to be sinteredwith microwaves comprising: an insulating wall permeable to microwaves,wherein the insulating wall defines a sintering chamber and the objectto be sintered are disposed in the sintering chamber; a microwavegenerator for radiating microwaves to the object to be sintered via theinsulating wall; and a feeding system for feeding the object to besintered into the sintering chamber, wherein the temperature within thesintering chamber is changed in order to correspond to a sinteringprocess of the object to be sintered in a feeding direction of theobject to be sintered by changing the thickness of the insulating wallin the feeding direction.
 2. The continuous sintering furnace accordingto claim 1, wherein the temperature within the sintering chamber ischanged in order to correspond to the sintering process of the object tobe sintered in the feeding direction of the object to be sintered bychanging adiabatic performance of microwave absorptance of theinsulating all in the feeding direction.
 3. The continuous sinteringfurnace according to claim 1, wherein the sintering chamber comprises aninlet and an outlet facing the inlet and the feeding system feeds theobject to be sintered from the inlet through the sintering chamber tothe outlet.
 4. The continuous sintering furnace according to claim 3,wherein the thickness of the insulating wall increases gradually fromthe inlet toward the outlet.
 5. The continuous sintering furnaceaccording claim 1, wherein the sintering chamber is formed in the shapeof a circular arc.
 6. The continuous sintering furnace according toclaim 1, wherein the feeding system has a furnace bed disposed below thesintering chamber on which the object to be sintered is disposed and theobject to be sintered is fed by rotation of the furnace bed.
 7. Thecontinuous sintering furnace according to claim 1, wherein the sinteringchamber is formed in the shape of a straight line.
 8. the continuoussintering furnace according to claim 1, wherein the feeding systemincludes a carriage having a mount portion on which the object to besintered is disposed and a roller coupled to the mount portion in orderto move the carriage, and the object to be sintered is disposed on themount portion and carried by movement of the carriage.
 9. The continuoussintering furnace according to claim 1, further comprising, within theinsulating wall, an inner shell which is able to heat itself withmicrowaves; wherein the inner wall surrounds the object to be sintered.10. The continuous sintering furnace according claim 1, furthercomprising, within the insulating wall, an inner shell which is able toheat itself with microwaves; wherein the inner wall surrounds the objectto be sintered, and the amount of heat generated with the microwaves perunit volume of the inner shell is larger than the amount of heatgenerated per unit volume of the object to be sintered, and also thetemperature of the inside surface of the inner shell is substantiallythe same as the surface temperature of the object to be sintered. 11.The continuous sintering furnace according to claim 1, furthercomprising a pre-treatment chamber disposed to be in parallel with thesintering chamber; wherein the object to be sintered disposed in thepre-treatment chamber is dried or baked using no glaze with radiationheat or transmitting microwaves from the outer surface of the insulatingwall originated from microwaves irradiated into the sintering chamber bythe microwave generator.